This invention relates to a vacuum circuit breaker in which an improved coil electrode is provided for magnetically driving an arc generated during current interrupting operation.
An axial magnetic field type electrode is commonly employed in a vacuum circuit breaker designed for interrupting a large current. This axial magnetic field type electrode includes a coil electrode provided on an outer end of each of a pair of arc electrodes. When one of the arc electrodes is parted away from the other arc electrode to interrupt the flow of current, an arc jumps across the two arc electrodes. Arc current sustaining the arc flows through the coil electrodes into rods connected to the coil electrodes, and the current flowing through the coil electrodes produces a parallel (axial) magnetic field. By application of this parallel magnetic field to the arc, the arc is divided into numerous filament-like arcs and is finally extinguished. However, because the coil electrodes employed hitherto in such an electrode arrangement have a long current path which leads to a high electrical resistance, the coil electrodes generate a large quantity of heat even in the normal current conducting condition. Thus, the electrode arrangement is defective in that the larger the current conduction capacity, more heat is generated.
With a view to solve the above problem, JP-A No. 56-63723 and JP-A No. 56-118227 propose an improved axial magnetic field type electrode. According to the disclosures of these publications which solve the above problem, current is conducted through arc electrodes in the normal conducting condition, while current is conducted through a coil electrode only in the current interrupting condition.
That is, the proposed electrode arrangement includes a cup-shaped outer electrode mounted on a stationary rod, a movable rod disposed inside the outer electrode, a cup-shaped inner electrode mounted on the inner end of the movable rod, and a contact and a coil electrode mounted on one and the other end respectively of the inner electrode. An arc gap is formed between the inner electrode and the outer electrode. When the movable rod is moved toward the stationary rod to bring the contact into contact with the outer electrode thereby closing the circuit, current flows through the route of the movable rod - the inner electrode - the contact the outer electrode - the stationary rod in the normal current conducting condition. On the other hand, when the movable rod is moved in the direction opposite to that described above to break the circuit, current flows through the route of the stationary rod--the outer electrode--an arc jumping across the arc gap between the outer and inner electrodes--the inner electrode the coil electrode, and a parallel magnetic field produced by the coil electrode is applied to the arc in parallel to the arc.
However, because of the provision of the cup-shaped inner and outer electrode, the proposed electrode arrangement is defective in that its structure is complex.